Transition metal complexes having alkylphosphine compounds as ligands are very important catalysts in carbon-carbon bond forming reactions such as Suzuki-Miyaura reaction, carbon-nitrogen bond forming reactions such as Buchwald-Hartwig amination, and carbon-oxygen bond forming reactions such as ether synthesis (see Nonpatent Document 1).
As an example, bis(tri-tert-butylphosphine)palladium (0) is used.
Many of the transition metal complexes having alkylphosphine ligands are very expensive, and the industrial availability thereof is low. Further, synthesis of the transition metal complexes having alkylphosphine ligands is difficult because the raw-material alkylphosphine compounds are generally extremely susceptible to air oxidation and possess combustibility.
For such reasons, the alkylphosphine compounds are used together with transition metals, salts thereof, oxides thereof or complexes thereof in the reaction system, in place of the isolated transition metal complexes having alkylphosphine ligands (see Nonpatent Documents 1 and 3). For example, di-tert-butylmethylphosphine, tri-tert-butylphosphine or tricyclohexylphosphine is used together with palladium (II) acetate or tris(dibenzylideneacetone)dipalladium (0) in the reaction system.
However, many of the alkylphosphine compounds are extremely susceptible to air oxidation and possess combustibility, and therefore are difficult to handle.
To improve the susceptibility to air oxidation, alkylphosphonium tetrafluoroborates, quaternary salts of alkylphosphines and boron compounds, have been studied. Examples of the alkylphosphonium tetrafluoroborates include:
(1) triethylphosphonium tetrafluoroborate (see Nonpatent Document 2);
(2) tricyclohexylphosphonium tetrafluoroborate (see Nonpatent Document 4);
(3) di-tert-butylmethylphosphonium tetrafluoroborate (see Nonpatent Document 3);
(4) tri-n-butylphosphonium tetrafluoroborate (see Nonpatent Document 5); and
(5) tri-tert-butylphosphonium tetrafluoroborate (see Nonpatent Document 4).
These compounds are produced from alkylphosphine compounds and fluoroboric acid (see Nonpatent Document 5).
As known in the art, the above compounds are used together with transition metals, salts thereof, oxides thereof or complexes thereof in the carbon-carbon bond forming reactions such as Suzuki-Miyaura reaction (see Nonpatent Documents 3 and 5). For example, di-tert-butylmethylphosphonium tetrafluoroborate or tri-tert-butylphosphonium tetrafluoroborate is used together with palladium (II) acetate, tris(dibenzylideneacetone)dipalladium (0) or bis(benzonitrile)dichloropalladium (II) in the reaction system.
Fluoroboric acid used as raw material in the production of the compounds (1) to (5) are corrosive and penetrate into the skin upon contact, and must be handled carefully. Furthermore, fluoroboric acid has acidity to corrode production utility made of stainless steel, and when hydrofluoric acid is liberated, it will corrode production utility made of glass. Therefore, the actual use of the above compounds in the production causes problems.
Alkylphosphonium tetraarylborate compounds are also developed, and the following compounds are known.
(6) triethylphosphonium tetraphenylborate (see Patent Document 1);
(7) tri-n-butylphosphonium tetraphenylborate (see Patent Document 1 and Nonpatent Document 6);
(8) tricyclohexylphosphonium tetraphenylborate (see Nonpatent Documents 4 and 7); and
(9) tri-tert-butylphosphonium tetraphenylborate (see Nonpatent Documents 4 and 7).
Nonpatent Documents 4, 6 and 7 describe the production of the alkylphosphonium tetraarylborate compounds. Specifically, the documents describe the following production processes (10) to (12).
(10) Tricyclohexylphosphine is reacted with fluoroboric acid to synthesize tricyclohexylphosphonium tetrafluoroborate, which is reacted with sodium tetraphenylborate to produce tricyclohexylphosphonium tetraphenylborate (75% yield). A similar process is described in which tri-tert-butylphosphine is used as starting material to produce tri-tert-butylphosphonium tetraphenylborate (71% yield) (see Nonpatent Document 4).
(11) Tri-tert-butylphosphine is reacted with 1,1,1,3,3,3-hexafluoro-2-propanol and with sodium tetraphenylborate to produce tri-tert-butylphosphonium tetraphenylborate (77% yield). A similar process is described in which tricyclohexylphosphine is used as starting material to produce tricyclohexylphosphonium tetraphenylborate (77% yield) (see Nonpatent Document 7).
(12) Tri-n-butylphosphine is reacted with hydrochloric acid in the presence of sodium tetraphenylborate to produce tri-n-butylphosphonium tetraphenylborate (53% yield) (see Nonpatent Document 6).
The four compounds (6) to (9) are the only compounds known as the alkylphosphonium tetraarylborate compounds, and the three processes (10) to (12) are the only known processes for producing them.
The processes (10) (Nonpatent Document 4) use fluoroboric acid and consequently have handling problems and problems of corrosion of production facility, and are not suited for industrial production.
The processes (11) (Nonpatent Document 7) use 1,1,1,3,3,3-hexafluoro-2-propanol which is expensive, and are not suited for industrial production. More inexpensive processes are desirable.
In the process (12) (Nonpatent Document 6) in which tri-n-butylphosphine is reacted with hydrochloric acid in the presence of sodium tetraphenylborate, the yield of tri-n-butylphosphonium tetraphenylborate is low (53% in terms of tri-n-butylphosphine). The reason for the low yield is not clear but is probably that a side reaction takes place between the reaction product of sodium tetraphenylborate with hydrochloric acid, and tri-n-butylphosphine.
The documents recited above do not describe that the carbon-carbon bond forming reactions, carbon-nitrogen bond forming reactions and carbon-oxygen bond forming reactions wherein the transition metal complexes having phosphine ligands produce catalytic effects, may be catalyzed by phosphonium tetraarylborate compounds together with transition metals, salts thereof, oxides thereof or complexes thereof in place of the transition metal complexes having phosphine ligands.
Thus, there is a need for the development of alkylphosphine derivatives that are producible without special reaction equipment and by simple operations, and have good handling properties.    Patent Document 1: JP-A-S62-149721 (pp. 2 and 3) Nonpatent Document 1: Journal of American Chemical    Society (U.S.A.) (2000, vol. 122, No. 17, pp. 4020-4028) 5 Nonpatent Document 2: Catalog of Strem Chemicals, Inc.    Nonpatent Document 3: Journal of American Chemical Society (U.S.A.) (2002, vol. 124, No. 46, pp. 13662-13663)    Nonpatent Document 4: Journal of American Chemical Society (U.S.A.) (1991, vol. 113, No. 3, pp. 875-883)    10 Nonpatent Document 5: Organic Letters (U.S.A.) (2001, vol. 3, No. 26, pp. 4295-4298)    Nonpatent Document 6: Organometallics (U.S.A.) (1999, vol. 18, No. 20, pp. 3981-3990)    Nonpatent Document 7: Journal of American Chemical 15 Society (U.S.A.) (1997, vol. 119, No. 16, pp. 3716-3731)
It is an object of the present invention to provide a novel process whereby a phosphonium borate compound is produced safely on an industrial scale, by simple reaction operations and in a high yield. It is another object of the invention to provide a novel phosphonium borate compound that is easily handled. It is a further object of the invention to provide a novel use of the phosphonium borate compound in combination with a transition metal, salt thereof, oxide thereof or complex thereof in the carbon-carbon bond forming reactions, carbon-nitrogen bond forming reactions and carbon-oxygen bond forming reactions wherein a transition metal complex having a phosphine ligand produces catalytic effects, wherein the phosphonium borate compound in combination with the transition metal, salt thereof, oxide thereof or complex thereof is used in place of the transition metal complex having a phosphine ligand.